The prior art is described below with help of following figures.
FIG. 1 shows the sectional view of the channel header assembly of typical H-H type heat exchanger as described bellow.
FIG. 2 shows the sectional view of the channel header assembly of typical H-L type heat exchanger as described bellow.
Threaded channel closure type heat exchangers are generally classified based on the operating pressures on the shell and tube sides.
The heat exchangers, which have high pressure on both, shell side as well as tube side, are classified as H-H type heat exchangers.
The heat exchangers, which have higher pressure on channel side and lower pressure on shell side, are classified as H-L type heat exchangers.
Thus in H-H type heat exchangers, tubesheets are subjected to lesser differential pressure and consequently would typically have internal tubesheet with apparatus for sealing the tubesheet against the shoulder of the channel.
In H-L type heat exchangers, the tubesheet can typically get exposed to full pressure of either side independently. H-L type would typically have tubesheet and channel of integral construction, either single piece or welded together. The shell of the H-L type could be independently bolted or welded to the tubesheet.
Tubesheets are provided with plurality of holes in which the tubes 5 are fixed. The channel (1) is provided with nozzles (6) for the tube side fluid to enter and exit the heat exchanger. The heat exchangers are preferably provided with two or more tube passes and accordingly, some of the tubes are in the first tube pass through which the tube side fluid enters the tube bundle from the channel side of the nozzle, while some tubes are in the final tube pass through which the tube side fluid exits from the tube bundle to the channel side exit nozzle.
Both H-H and H-L type heat exchangers have channel headers (1) provided with a threaded closure consisting of threaded lock ring (2) and channel cover (3). Threaded lock ring is screwed in the threads provided in the channel header body.
The closure is sealed by means of a gasketed joint. The gasket (7) is located in a groove made in shoulder of the channel ahead of the threads. The gasket is compressed through the peripheral portion i.e. the tongue of the diaphragm (8). The diaphragm (8) is backed by a compression ring (9) at the periphery; and at the center, it is backed by channel cover. The channel cover is held in position by the threaded lock ring (2). The push bolts/rods (10) assembled at the periphery of the threaded lock ring pressurize the compression ring (9), which in turn presses tongue of the diaphragm to seal the gasket. The end thrust due to internal pressure on diaphragm is essentially transmitted to and resisted by the channel threads via channel cover, outer compression ring and threaded lock ring. The push bolts/rods (10) on the threaded lock ring (2) provide incremental loading to the gasket through the tongue of the diaphragm for achieving the leak-tight joint.
The assembly procedure of the prior art heat exchanger is as given below.
The diaphragm (8) and the outer compression ring (9) are placed in position. The diaphragm (8) and compression ring (9) are held in position by means of internal grub screws (11). These grub screws are screwed in the radial threaded holes provided in the outer compression ring (9). These grub screws are assembled from inner side of the outer compression ring and project beyond the outside diameter of outer compression ring to get engaged in the dimples (51) provided in the channel barrel. These grub screws remain under flush with respect to the inside diameter of outer compression ring to avoid interfering with the channel cover. After this, the channel cover (3) and threaded lock ring can be assembled.
It should be noted that the heat exchangers under consideration are very heavy and the size and weight of the components like channel cover and the threaded lock ring can be in the range of 600 mm to 2000 mm in diameter and weights in the range of 200 kg to 10000 kg. Naturally, such parts require liberal clearances between their mating parts (typically of the order of 0.5 mm to 2 mm radially) to facilitate the assembly. The arrangement of the closure is such that the major portion of the cover gets located in the inside diameter of the threaded lock ring whereas a small portion of the channel cover, whose diameter is bigger than the inside diameter of the threaded lock ring remains ahead of the threaded ring and enters into the outer compression ring. Therefore, the threaded lock ring can be assembled only when the channel cover is in position. At the same time, the channel cover cannot remain in position without being engaged with the threaded lock ring. Hence, the existing art is to handle both the threaded lock ring and the channel cover together for assembly into the channel header. Hence, it is necessary to handle the threaded lock ring and the channel cover together. These components are mounted on a special cantilever type fixture and are required to be skillfully balanced using counterweights to make them vertical and aligned with the centerline of the channel. Further, during assembly, while the cover remains stationary, the threaded lock ring is required to be rotated. The existing methodology of balancing the assembly, alignment of the threaded lock ring with the threads on the channel body is very difficult, cumbersome and time consuming process and if not done properly can severely damage the equipment and/or its components.
The grub screws (11), which are screwed from inside of the outer compression ring (9) can hamper free movement of the compression ring (9) and the diaphragm (8). After equipment has been exposed to operating condition, invariably these grub screws (11) get jammed and become extremely difficult to unscrew, more so because these screws are under flush with outer compression ring and at times get broken leading to major repair of the equipment.
There is definitive tendency for the push bolts to get jammed particularly when the equipment has been in service for some period of time due to deposition of extraneous matter, scaling, rusting and/or galling in the threaded portion. When the torque is applied for loosening of such jammed bolts, it is very likely that the heads of the bolts are sheared off. The removal and replacement of these sheared bolts is very difficult and cumbersome and not always possible at the site of installation.
As mentioned above, generous clearances are provided between the mating roots and crests of the male threads on the threaded lock ring (2) and female threads on the channel barrel to facilitate smooth assembly. Ideally, it is required that, this clearance should remain distributed uniformly and equally around the circumference so that the load transfer takes place along the contact at side of the flanks of the threads along the pitch line of the threads. However, due to the self weight, there is a natural tendency for the threaded lock ring (2) to settle down towards the bottom portion. This could induce differential force around the circumference leading to eccentric forces on the threads, leading to very high contact stresses in the bottom side of the thread flanks compared to that on the top side. This could lead to scoring/scrapping of the metal during unscrewing, especially after the heat exchanger has been exposed to operating conditions for some time, leading to jamming of the threads and can cause severe damage to the equipment.